Semiconductor devices are fabricated by depositing and patterning one or more conductive, insulating, and semiconductor layers to form integrated circuits. Some integrated circuits have multiple layers (or multilevels) of interconnect, which results in the ability to manufacture more die per wafer. The top interconnect level comprises or is connected to bond pads, which are used to connect the semiconductor devices to other die, e.g., in a multi-chip module, or to leads of a package, as examples.
As semiconductor devices are scaled down in size, the propagation delay, or the RC delay, becomes a concern. To reduce this delay, there is a trend in the semiconductor industry towards the use of copper for interconnect layers of semiconductor devices, to reduce the capacitance between conductive lines. Integrated circuit (IC) technology has migrated into copper (Cu) trace circuits with 0.15 μm, 0.13 μm or even more advanced generic schemes using a top metal pad using copper metal instead of aluminum (Al) bond pads, as was common in prior semiconductor technology.
However, copper oxidation linearly grows through the inner copper matrix and is not self-limiting as is the case with aluminum bond pads, since dense aluminum oxide prevents moisture from penetrating therethrough and permitting continued aluminum oxidation. Thus, an aluminum cap is sometimes formed over copper traces, and wires are then bonded directly to the aluminum cap. Not only does aluminum form a self-limiting oxide, often it is difficult to wire bond directly to copper, and having an aluminum cap over copper pads makes it easier to wire bond to an integrated circuit.
FIG. 1 shows a cross-sectional view of prior art aluminum caps 106 and 108 formed over copper lines or bond pads 112 and 114. A workpiece 110 having a plurality of copper bond pads 112 and 114 formed thereon within an insulating layer 102 is provided. The aluminum caps 106 and 108 extend over the edge regions of a top passivation layer 104 proximate the aluminum caps 106 and 108, as shown. The aluminum caps 106 and 108 formed over the copper bond pads 112 and 114 protect the copper bond pads 112 and 114 from oxidation and also facilitate a wire bond connection. Prior art aluminum caps 106 and 108 are typically formed upon pure copper bond pads 112 and 114, and extend over or overhang a portion of the adjacent passivation layer 104, e.g., at 107 and 109.
A problem with the prior art aluminum caps 106 and 108 is that the exposed aluminum caps 106 and 108, particularly at the overhangs 107 and 109, are easily scratched during transportation and quality control (QC) inspection handling, as examples, and may be distorted after backside grinding or wire bonding, which induces pad bridging issues, leading to electrical failures/shorts, e.g., at 111, as shown in a top view in FIG. 2. This occurs because the aluminum caps 106 and 108 are not protected by any overlying insulating or passivation layer and are thus easily scratched or damaged during normal handling/processing. The dislodged aluminum cap 106 and 108 material creates bridges and shorts 111 between adjacent bond pads 112 and 114, as shown.
Thus, what is needed in the art is a method of forming aluminum caps over bond pads of a semiconductor device in which shorts do not form between adjacent bond pads.